---
title: Async Queries via Celery
hide_title: true
sidebar_position: 9
version: 1
---

## Async Queries via Celery

### Celery

On large analytic databases, it’s common to run queries that execute for minutes or hours. To enable
support for long running queries that execute beyond the typical web request’s timeout (30-60
seconds), it is necessary to configure an asynchronous backend for Superset which consists of:

- one or many Superset workers (which is implemented as a Celery worker), and can be started with
  the `celery worker` command, run `celery worker --help` to view the related options.
- a celery broker (message queue) for which we recommend using Redis or RabbitMQ
- a results backend that defines where the worker will persist the query results

Configuring Celery requires defining a `CELERY_CONFIG` in your `superset_config.py`. Both the worker
and web server processes should have the same configuration.

```python
class CeleryConfig(object):
    broker_url = "redis://localhost:6379/0"
    imports = (
        "superset.sql_lab",
        "superset.tasks.scheduler",
    )
    result_backend = "redis://localhost:6379/0"
    worker_prefetch_multiplier = 10
    task_acks_late = True
    task_annotations = {
        "sql_lab.get_sql_results": {
            "rate_limit": "100/s",
        },
    }

CELERY_CONFIG = CeleryConfig
```

To start a Celery worker to leverage the configuration, run the following command:

```
celery --app=superset.tasks.celery_app:app worker --pool=prefork -O fair -c 4
```

To start a job which schedules periodic background jobs, run the following command:

```
celery --app=superset.tasks.celery_app:app beat
```

To setup a result backend, you need to pass an instance of a derivative of from
from flask_caching.backends.base import BaseCache to the RESULTS_BACKEND configuration key in your superset_config.py. You can
use Memcached, Redis, S3 (https://pypi.python.org/pypi/s3werkzeugcache), memory or the file system
(in a single server-type setup or for testing), or to write your own caching interface. Your
`superset_config.py` may look something like:

```python
# On S3
from s3cache.s3cache import S3Cache
S3_CACHE_BUCKET = 'foobar-superset'
S3_CACHE_KEY_PREFIX = 'sql_lab_result'
RESULTS_BACKEND = S3Cache(S3_CACHE_BUCKET, S3_CACHE_KEY_PREFIX)

# On Redis
from flask_caching.backends.rediscache import RedisCache
RESULTS_BACKEND = RedisCache(
    host='localhost', port=6379, key_prefix='superset_results')
```

For performance gains, [MessagePack](https://github.com/msgpack/msgpack-python) and
[PyArrow](https://arrow.apache.org/docs/python/) are now used for results serialization. This can be
disabled by setting `RESULTS_BACKEND_USE_MSGPACK = False` in your `superset_config.py`, should any
issues arise. Please clear your existing results cache store when upgrading an existing environment.

**Important Notes**

- It is important that all the worker nodes and web servers in the Superset cluster _share a common
  metadata database_. This means that SQLite will not work in this context since it has limited
  support for concurrency and typically lives on the local file system.

- There should _only be one instance of celery beat running_ in your entire setup. If not,
  background jobs can get scheduled multiple times resulting in weird behaviors like duplicate
  delivery of reports, higher than expected load / traffic etc.

- SQL Lab will _only run your queries asynchronously if_ you enable **Asynchronous Query Execution**
  in your database settings (Sources > Databases > Edit record).

### Celery Flower

Flower is a web based tool for monitoring the Celery cluster which you can install from pip:

```python
pip install flower
```

You can run flower using:

```
celery --app=superset.tasks.celery_app:app flower
```
